Characteristics of fluid inclusions and metallogenic mechanism of the Longshang skarn type tin polymetallic deposit in eastern Hunan Province, China

Global tin mineral resource is mainly derived from skarn-type Sn deposits, however, its formation mechanism remains highly debated. Longshang skarn-type Sn deposit, located in the middle part of Xitian Sn polymetallic orefield, eastern Hunan Province, is a typical skarn-type Sn deposit in the world famous Nanling W-Sn metallogenic blet. Based on geological observation, ore structure study and fluid inclusions investigation, we conduct systematic study upon fluid inclusions from different ore -forming stages of this deposit by using the method of fluid inclusion assemblage. The results show that the mineralization of Longshan skarn-type deposit can be divided into skarn, retrograde, greisen-oxide, quartz -sulfide and fluorite -carbonate stages, with the cassiterite forming mainly in the greisen-oxide stage. Fluid inclusions in quartz coexisting with cassiterite of the deposit include liquid -rich and vapor-rich two-phase fluid inclusions, liquid CO2-bearing three-phase fluid inclusions and pure CO2 fluid inclusions. The homogenization temperature and salinity of the fluid inclusions have a relatively large range of 200 similar to 400 degrees C and 2% similar to 9% NaCleqv, indicating the ore -forming fluid experienced immiscibility. The fluid inclusions in quartz coexisting with sulfide is mainly liquid -rich ones with a temperature and salinity range of 190 similar to 261 degrees C and 3% -7% NaCleqv, suggesting the fluid experienced cooling and immiscibility. Our studies suggest that fluid immiscibility is the major reason that leads to the formation of Sn deposit in the greisen-oxide stage, while fluid cooling and immiscibility is the major mechanism for the formation of sulfide.